doc, af_xdp: Fix bind flags option typo

Fix XDP_ZERO_COPY flag typo since it is actually named XDP_ZEROCOPY
instead as per if_xdp.h uapi header.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/1656fdf94704e9e735df0f8b97667d8f26dd098b.1625550240.git.baruch@tkos.co.il